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How do I figure out what is the oxidizing agent and the reducing agent in the following reaction:

$$\ce{\overset{-3}{N}\overset{+1}{H}_3 + \overset{+1}{H}\overset{-2}{O}\overset{+1}{Cl} <=> \overset{-1}{N}\overset{+1}{H}_2\overset{-1}{Cl} + H2O}~?$$

I assigned known oxidation numbers for the elements, but I'm not sure how to proceed with the rest.

Is $\ce{NH3}$ the reducing agent? Or nitrogen? Going from $-3$ to $-1$ and $\ce{HOCl}$ the oxidizing agent, chlorine going from $+1$ to $-1$?

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You're on the right track with assigning oxidation numbers to each element. And you've assigned them successfully.

To successfully complete the task you need to define a few terms:

What's oxidation?

What's reduction?

What's the oxidizing agent? Try thinking of this in terms of what oxidation is, and then try figuring out what an oxidizing agent would do.

What's the reducing agent?

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  • $\begingroup$ And what is a Redox Reaction? $\endgroup$ – Devashish Das Aug 13 '14 at 4:09
  • $\begingroup$ ^Good question to be asking oneself. $\endgroup$ – Dissenter Aug 13 '14 at 4:11
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    $\begingroup$ Thank you! Oxidation would be the loss of at least one electron, and reduction is the gain of at least one. Oxidizing agent is the one that is reduced because it lets the other one get oxidized. So if Nitrogen in NH3 is going from -3 to -1, it is losing electrons (since charge is increasing) meaning it is oxidized, making it the reducing agent? $\endgroup$ – user3325170 Aug 13 '14 at 4:32
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As Dissenter had already pointed out, your assignment of oxidation states of each element is spot-on. Through your comments you seem to know that-

  • Oxidation refers to the loss of electrons or an increase in the oxidation state of an atom by another atom, an ion, or a molecule.
  • Reduction refers to the gain of electrons or a decrease in the oxidation state of an atom by another atom, an ion, or a molecule.
  • Substances that have the ability to oxidize other substances (cause them to lose electrons) are known as oxidizing agents, as they remove electrons from another substance, and thus itself get reduced. As it accepts electrons, it's also called an electron acceptor. Vice versa for a reducing agent.

(For more details you can check out the Wiki page on redox reactions.)

For deducing which is the oxidizing agent and why is it so we come to the actual picture. The change of oxidation state of $\ce{N}$ from -3 to -1 indicates it being oxidized as it loses 2 electrons (and hence the negative charge on it decreases). Thus by the definition above, $\ce{NH3}$ is the reducing agent as it gets oxidized itself (as $\ce{N}$ goes from -3 to -1).

While change of oxidation state of $\ce{Cl}$ is from +1 to -1, this shows that it has been reduced as it has gained 2 electrons (and hence the charge on it changes from positive to negative). Thus $\ce{HOCl}$ is the oxidizing agent as it gets reduced itself (as $\ce{Cl}$ goes from +1 to -1).

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